The popularity of racquet sports has increased significantly over the past 20 years. This has been due, at least in part, to advances in racquet design. For example, in the 1970's Prince.RTM. introduced the large head tennis racquet, based on its U.S. Pat. No. 3,999,756, which made the game easier and more enjoyable to play. Since that time, advances in lightweight composite materials, and the introduction of widebody frame profiles, have allowed racquet frames to become lighter and stiffer, which allows players of all skill levels to hit the ball with greater speed and accuracy and thus derive even more enjoyment from the game.
The larger head of modern day tennis racquets also provides a greater ability for players to hit spin shots, and tennis instructors increasingly teach spin shots as a way to improve the player's game. Thus, rather than hitting a "flat" shot, in which the racquet head merely moves forward to meet the ball, a player swings the racquet so that, at the time of ball impact, the racquet head is also moving upwardly (to impart a topspin), or downwardly (to impart an underspin), or sideways (to impart a slice), or various combinations thereof.
Balls hit with a spin are harder to return because they curve during flight and because the spin alters the angle at which the ball bounces. Because an opponent does not know how the ball is spinning, it is more difficult for opposing players to anticipate the flight and bounce, and position themselves properly to hit a strong return, than in the case of a flat shot.
Topspin ground strokes are particularly desirable. Topspin tends to cause the ball to curve downwardly toward the opponent's court. Thus, it is easier to hit shots deep, because the spin helps ensure that the ball will land inside the baseline. Moreover, when a ball hit with topspin bounces, it tends to kick forward, due to the spin, which means that opponents need to retreat deeper to hit the return. The fact that an opponent will be hitting a deep return means that a topspin shot, placed deep, does not need to be hit hard to be effective, which in turn makes topspin shots easier to hit.
FIG. 1 illustrates the execution of a topspin forehand. The player swings the racquet forward so that, as the ball approaches, the racquet is below the level of the ball, as shown by position "A". The swing motion continues by moving the racquet in a forward and upward direction, as shown by the arrow, from positions "A" to "C", to intersect the path of movement of the ball. During the swing, the racquet head remains generally in a vertical position, so that, at the moment of ball impact, when the racquet has reached position "B", the forward component of racquet movement causes the ball to rebound in the direction of the opponent's court, while at the same time, the upward component of racquet movement imparts a clockwise (looking in the direction of FIG. 1) topspin.
The widebody design frame, in which the cross-sectional height is substantially greater than the cross-sectional width, produces higher out-of-plane bending stiffness (i.e., bending stiffness in a direction perpendicular to the string bed), which results in increased hitting power when hitting a flat serve or return. However, the increased frame height is a disadvantage when attempting to hit a spin shot.
One reason has to do with the fact that, in a spin shot, the path of movement of the frame needs to cross the path of movement of the ball. As shown in FIG. 2, in order for the ball to land on the strings, just prior to impact the upper side of the frame needs to cross the path of the ball, but at the time of impact the lower side of the frame must be below the ball path.
However, as shown by FIG. 3, which illustrates the movement of the ball relative to the string bed, just prior to impact, the ball must have a certain minimum angle of incidence e in order to avoid hitting the leading edge "le" of the frame. As can be seen from FIG. 3, the relatively large cross-sectional height "h" of a widebody frame increases the minimum necessary angle of incidence, making it harder to land the ball on the strings, during a spin shot, without hitting or grazing the sides of the frame. For example, in a racquet frame having a cross-sectional height "h" of 20 mm, an outside width of 285 mm, and a cross-sectional frame width of 8 mm, the minimum angle of incidence .alpha. would be 4.1.degree., assuming one could land the ball directly in the center "C" of the string bed. If the ball lands either above or below center "C", the angle .alpha. must be even greater (note that although a ball that hits the strings below center "C" could theoretically have an angle of incidence less than 4.degree. and still miss the top of the frame, upon rebounding from the string bed it would hit the trailing edge "te" of the frame; thus the minimum angle of incidence must allow the ball both to hit the strings and rebound off the racquet without hitting the frame).
Another tradeoff of a widebody cross-section is that it increases the wind resistance of the racquet when attempting to hit spin shots. As described above, a spin shot utilizes a stroke in which the racquet head moves with a substantial component of motion parallel to the strings, i.e., tangential to tip of the frame head. During tangential motion (as opposed to motion perpendicular to the string bed), the broad side of the widebody profile faces into the wind, presenting a relatively large surface area. Because it is desirable to obtain maximum tangential head speed for spin production, the surface area of the side of the frame is extremely important. If the total surface area facing into the wind is large, as is the case with widebody frames, tangential head speed will be impaired.